Highway departments use a variety of techniques to monitor traffic in an effort to detect, mitigate, and prevent congestion. Typically, each highway department has a command center that receives and integrates a plurality of signals transmitted by monitoring systems located along the highway. Although different kinds of monitoring systems are used, the most prevalent employs a roadway metal detector. A wire loop is embedded in the roadway and its terminals are connected to detection circuitry that measures the inductance changes in the wire loop. Because the inductance in the wire loop is perturbed by a motor vehicle (comprising a quantity of ferromagnetic material) passing over it, the detection circuitry can detect when a motor vehicle is over the wire loop. Based on this perturbation, the detection circuitry creates a binary signal, called a "loop relay signal," which is transmitted to the highway department's command center. The command center gathers the respective loop relay signals and from them makes a determination as to the likelihood of congestion. The use of wire loops is, however, disadvantageous for several reasons.
First, a wire loop system will not detect a motor vehicle unless the motor vehicle comprises sufficient ferromagnetic material to create a noticeable perturbation in the inductance in the wire loop. And because the trend is to fabricate motor vehicles with non-ferromagnetic alloys, plastics and composite materials, wire loop systems will increasingly fail to detect the presence of motor vehicles. It is already well known that wire loops often overlook small vehicles. Another disadvantage of wire loop systems is that they are expensive to install and maintain. Installation and repair require that a lane be closed, that the roadway be cut and that the cut be sealed. Often too, harsh weather can preclude this operation for several months.